1. Field of the Invention
This invention relates generally to superconducting dynamoelectric machines, and more specifically, this invention relates to a superconducting dynamoelectric machine having an improved arrangement for supporting a cryogenic temperature portion of the rotor on an ambient temperature portion of the rotor.
2. Description of the Prior Art
Superconducting dynamoelectric machinery having a rotating superconductive field winding requires a stable support for the cryogenic portion of the rotor, while still limiting heat conduction from the ambient temperature portion of the rotor to the greatest extent possible. In prior art cryogenic structures (Dewar vessels), an outer ambient temperature structure has a fill tube passing through it to convey the cryogenic material to an inner cryogenic temperature portion. The fill tube is affixed to the ambient temperature structure to provide support for the cryogenic temperature portion. In addition, thin wires or spokes are utilized to help support the cryogenic temperature portion. The cryogenic temperature portion is surrounded by a vacuum to eliminate convection losses and the surfaces of the inner and outer walls are highly polished to lower radiation losses. Heat conduction to the cryogenic temperature portion is directly proportional to the cross-sectional area divided by the length of the supports linking the ambient temperature and cryogenic temperature portions. Long thin supports are therefore used to reduce conduction losses, which would otherwise result in excessive "boil off" of the cryogenic material.
However, in a stationary Dewar vessel the supporting structure is only required to support the weight of the assembly and is not required to provide precision placement of the cryogenic temperature portion with respect to the ambient temperature portion. On the other hand, in a rotating Dewar assembly, such as a cryogenic portion of the superconducting machine rotor, the cryogenic temperature portion must be supported for static and dynamic loads. The superconducting field winding located in the cryogenic temperature portion may have a significant mass. Thus, the supporting arrangement for the cryogenic temperature portion must: transmit machine torque from the field winding to the prime mover; maintain the concentricity of the ambient temperature and cryogenic temperature portions; absorb axial thermal distortion; and, limit heat losses to the cryogenic temperature portion. One way to accomplish this is to use a tubular support for one end of the cryogenic temperature portion to provide torque transmittal capability, while the other end is supported by spokes to absorb axial thermal distortion and limit heat losses. The use of such a spoke supporting structure is illustrated, for example, in a copending application of D. C. Litz, Ser. No. 398,023, filed Sept. 17, 1973 now a Defensive Publication No. T 934,001 and assigned to the assignee of the present invention.
This supporting structure has been entirely satisfactory for rotors of relatively small size, that is, rotor structures having a ratio of length to diameter less than 5:1. When the ratio of the length to the diameter of the rotor assembly approaches or exceeds the ratio 5:1, the rotor body becomes quite flexible. This means that, as a result of its own weight, the body of the rotor is caused to sag or deflect downwardly, thus causing a relatively large downward curvature of the body. This large curvature in the body of the rotor, in combination with the variable moment of inertia of an area at right angles to the rotor body that is caused by the non-uniform density of the superconducting structure, causes two very serious modes of vibration to occur as the rotor is brought up to normal operating speed or as it is allowed to slow down to a standstill. One of these very serious modes of vibration takes place in a frequency range that substantially coincides with the frequency range of a main critical speed of the entire turbine generator, the speed being a speed in which the frequency of the whirling force is in resonance with a natural frequency vibration of the rotor assembly. The other serious mode of vibration takes place at a frequency which is substantially one-half of the main critical speed.
The entire support structure for the superconducting rotor assembly must be designed to have its resonances or critical speeds far enough removed from operating speed so that minimal shaft vibration occurs. This is particularly important for a cryogenic rotor for several reasons. Any vibrational energy produced in the rotor cold zone or surrounding structure will result in heat which must be removed by the refrigeration system. If excessive heat is produced, even momentarily, field quenching may occur. Secondly, the helium transfer system coupled to the exciter end of the rotor must run with extremely low vibration. Additionally, the physical air gap between the rotor member and the stator member will tend to be much smaller than on conventional machines. It is desirable, therefore, to provide a supporting structure for the cryogenic portion of the rotor so that it will operate either below the first critical speed or at a point intermediate of the first and second critical speeds. Such a structure should be adjustable so that the rotor member will not operate "close" to either critical point so that the maximum deflection of the rotor portion at the operational frequency does not exceed the maximum allowable deflection which can be tolerated under steady state conditions.